The present invention relates to an ink jet printing method and apparatus.
General Background to Inkjet Printing
Ink-jet printing is a non-impact dot-matrix printing technology in which droplets of ink are jetted from a small aperture directly onto a specified position on a medium, typically paper, to create an image. The mechanism by which a liquid stream breaks up into droplets was described by Lord Rayleigh in 1878. In 1951, Elmqvist of Seimens patented the first practical Rayleigh break-up ink-jet device. The development led to the introduction of the Mingograph, one of the first commercial ink-jet chart recorders for analog voltage signals. In the early 1960s, Dr. Sweet of Stanford University demonstrated that by applying a pressure wave pattern to an orifice, the ink stream could be broken into droplets of uniform size and spacing. When the drop break-off mechanism was controlled, an electric charge could be impressed on the drops selectively and reliably as they formed out of the continuous ink stream. The charged drops were deflected into a gutter by the electric field and were then recirculated. The uncharged drops were left to fly directly onto the media to form an image. The printing process described above is known as continuous ink-jet. By the late 1960s, Sweet's inventions led to the introduction of the A. B. Dick VideoJet and Mead DIJIT products. In the 1970s, IBM licensed the technology and launched a massive development program to adapt continuous ink-jet technology for their computer printers. The resulting IBM 4640 ink-jet printer was introduced in 1976 as a word processing hardcopy-output peripheral application.
At approximately the same time, Professor Hertz of the Lund Institute of Technology in Sweden and his associates independently developed several continuous ink-jet techniques that had the ability to modulate ink-flow characteristics for gray-scale ink-jet printing. One of Professor Hertz's methods of obtaining gray-scale printing was to control the number of drops deposited in each pixel. By varying the number of drops laid down, the ink volume in each pixel was controlled, thereby adjusting the density in each color to create the gray tone desired. The method produced commercial high-quality color images for the computer prepress color hardcopy market
While continuous ink-jet development was intense, the development of a drop-on-demand ink-jet method was also popularized. A drop-on-demand device ejects ink droplets only when they are used in imaging on the media. The on-demand approach eliminates the need for drop charging and deflection hardware, and also does away with inherently unreliable ink recirculation systems.
Zoltan, and Kyser & Sears, are among the pioneer inventors of the drop-on-demand ink-jet systems. Their inventions were used in the Seimens PT-80 serial character printer (1977) and by Silonics (1978). In these printers, on the application of voltage pulses, ink drops are ejected by a pressure wave created by the mechanical motion of a piezoelectric ceramic.
Many of the drop-on-demand ink-jet ideas and systems were invented, developed, and produced commercially in the 1970s and 1980s. The simplicity of the drop-on-demand ink-jet system was supposed to make ink-jet technology more reliable. However, during this period, the reliability of ink-jet technology remained poor. Problems such as nozzle clogging and inconsistency in image quality plagued the technology.
In 1979, Endo and Hara of Canon invented a drop-on-demand ink-jet method where ink drops were ejected from the nozzle by the growth and collapse of a water vapor bubble on the top surface of a small heater located near the nozzle. Canon called the technology the bubble jet The simple design of a bubble jet printhead, along with its semiconductor compatible fabrication process, allowed printheads to be built at low cost with high nozzle packing density. Apparently, during the same time period or shortly thereafter, Hewlett-Packard independently developed a similar ink-jet technology.
In 1984, Hewlett-Packard commercialized the ThinkJet printer, the first successful low-cost ink-jet printer based on the bubble jet principle, and named the technology thermal ink-jet. The cost of a ThinkJet printhead consisting of 12 nozzles was low enough that the printhead could be replaced every time the ink cartridge was empty. By replacing the print head each time, they had solved the reliability problem of ink-jet technology. Since then, Hewlett-Packard and Canon have continuously improved on the technology, and ink-jet printer models with higher printing resolution and color capability became available over the course of time at affordable prices. Since the late 1980s, because of their low cost, small size, quietness, and particularly their color capability, the thermal ink-jet or bubble jet printers became the viable alternative to impact dot-matrix printers for home users and small businesses. Currently, thermal ink-jet printers dominate the low-end color printer market.
Technology Map
Reference is now made to FIG. 1, which is a basic technology map that summarizes the various ink-jet technologies that are available. Ink-jet printing has been implemented in many different designs and has a wide range of potential applications. As shown in the figure, ink-jet printing is divided into the continuous and the drop-on-demand ink-jet methods.
Depending on the drop deflection methodology, the continuous ink-jet can be designed as a binary or multiple deflection system. In a binary deflection system, the drops are either charged or uncharged. The uncharged drops are allowed to fly directly onto the media, while the charged drops are deflected into a gutter for recirculation. In a multiple deflection system, drops are charged and deflected to the media at different levels. The uncharged drops fly straight to a gutter to be recirculated. This approach allows a single nozzle to print a small image swath. Both of these methods are widely used in the industrial coding, marking, and labeling markets. Products demonstrated include a 16.4 ft billboard size ink-jet printer that uses continuous ink-jet technology.
The majority of activity in ink-jet printing today, however, is in the drop-on-demand methods. Depending on the mechanism used in the drop formation process, the technology can be categorized into four major methods: thermal, piezoelectric, electrostatic, and acoustic. Most, if not all, of the drop-on-demand ink-jet printers on the market today use either the thermal or piezoelectric principle. Both the electrostatic ink-jet and acoustic ink-jet methods are still in the development stage with many patents pending and few commercial products available.
The thermal ink-jet method was not the first ink-jet method implemented in a product, but it is the most successful method on the market today. Two basic nozzle types are known for the thermal ink-jet, shown respectively in FIGS. 2 and 3. FIG. 2 shows the kind of nozzle known as a roof-shooter. In roof shooter nozzle 10, an orifice 12 for expulsion of droplet 14, is located above heater 16, where the upward direction is defined as being perpendicular to the plane in which the heater lies. In FIG. 3, an alternative nozzle, known as a side-shooter is shown. In the side shooter nozzle 18, an orifice 20 is located on a side near to heater 22, and substantially along the principle plane of the heater.
Reference is now made to FIG. 4, which is a simplified diagram illustrating four modes of a piezoelectric ink jet method. The heater of the nozzles of FIGS. 2 and 3 may be replaced by a piezoelectric crystal, which deforms in order to expel a drop of ink. Any one of four different piezoceramic deformation modes may be used, allowing the technology to be classified into four main types: squeeze, bend, push, and shear. The figure shows plus, zero and minus positions for three types of deformation, length and width, radial and shear.
Squeeze-mode ink-jet nozzles have been designed with a thin tube of piezoceramic surrounding a glass nozzle, and with a piezoceramic tube cast in plastic that encloses the ink channel. One version comprises a printhead array of twelve jets and an innovative maintenance station design. Subsequent efforts to introduce a second-generation printhead with a 32-jet array encountered difficulty in achieving jet-to-jet uniformity.
Reference is now made to FIG. 5, which is a simplified diagram illustrating a piezoelectric nozzle based on bend mode. In nozzle 30, one or more piezoceramic plates 32 are bonded to a diaphragm 34. The plates and the diaphragm together form an array of bilaminar electromechanical transducers which are used to eject ink droplets 36 via an orifice 38.
Reference is now made to FIG. 6, which is a simplified diagram showing a piezoelectric based nozzle for an ink jet printer which is based on a push-mode design. In nozzle 40, a piezoceramic rod pushes against diaphragm 44 at a point of contact 46 referred to as a foot. As the rod expands, under the influence of an excitation signal, it pushes the diaphragm against ink within the nozzle to eject droplets 48 via orifice 50. It will be appreciated that whilst a single rod is shown for simplicity, a practical nozzle may include a plurality of rods. In theory, piezodrivers, as the rods are referred to, can directly contact and push against the ink. However, in practice, the diaphragm is incorporated between the piezodrivers and the ink to prevent any undesirable interactions between ink and piezodriver materials.
In both the bend- and push-mode designs, the electric field generated between the electrodes is in parallel with the polarization of the piezoelectric material. Reference is now made to FIG. 7 which shows a nozzle for a shear-mode printhead. In shear mode nozzle 52 the electric field is designed to be perpendicular to the polarization of piezodriver 54. The shear action deforms the piezodrivers against the ink to eject the droplets 56 via orifice 58. In nozzle 52, the piezodriver becomes an active wall of ink chamber 60. Interaction between ink and piezomaterial is one of the key parameters of a shear-mode printhead design.
Printhead Design and Fabrication Processes.
Today the ink-jet technologies most active in laboratories and in the market are the thermal and piezoelectric drop-on-demand ink-jet methods. In a basic configuration, a thermal ink-jet consists of an ink chamber having a heater with a nozzle nearby. Reference is now made to FIGS. 8a . . . 8c which show three phases in the operation of such a basic configuration. In a first stage, FIG. 8a, a current pulse having a duration of less than a few microseconds is applied to heater 62, so that heat is transferred from the surface of the heater to ink 64 lying in chamber 66. The ink becomes superheated to the critical temperature for bubble nucleation. For water-based ink, the critical temperature is around 300° C. FIG. 8b shows nucleation occurring, wherein a water vapor bubble instantaneously expands to force ink out of the nozzle. Once all the heat stored in the ink is used, the bubble begins to collapse on the surface of the heater. Concurrently with the bubble collapse, the ink droplet breaks off as shown in FIG. 8c and accelerates towards the paper. The whole process of bubble formation and collapse typically takes place in less than 10 μs. The chamber is then replenished with ink and the process is ready to begin again. Depending on the channel geometry and the physical properties of the ink, the ink refill time can be from 80 to 200 μs.
Reference is now made to FIG. 9, which is a graph illustrating the process shown in FIG. 8 by plotting various parameters of the process including electrical pulse, temperature, pressure, and bubble volume against a common time axis. The graph shows the various pressure, temperature, and bubble volume changes during a thermal ink-jet drop formation cycle.
FIG. 10 shows a scanning electron microscope (SEM) photograph of a thermal ink-jet channel with heater and ink barrier layer. The jet supplied by the device in the photograph is known to produce ink droplets at the rate of 6000 drops per second. The ink channel in the SEM photograph measures approximately 0.025 mm thickness and a little more in width. However, the dimensional stability, accuracy, and uniformity of the channel are known to have significant effects on various performance features of the jet such as drop frequency, volume, and velocity. All of the performance parameters together ultimately determine the quality and throughput of the final printed image. The trends in the industry are currently to provide smaller droplets for image quality, faster drop frequency, and a higher number of nozzles for print speed, while the cost of manufacture is reduced.
The above manufacturing trends force further miniaturization of the ink-jet design. Consequently, the reliability issue becomes critical. In a recent generation of one popular ink jet series, a 192-nozzle tricolor printhead that can jet much smaller ink droplets (10 pl) at the rate of 12,000 drops per second was introduced. Ink feeds from both sides of the heater chamber. The fluid architecture significantly reduces the possibility of nozzle clogging from particulates. Particulates may for example have been trapped in the printhead fabrication processes or may be left in the ink from the ink manufacturing process. A row of small openings between the ink manifold and the heater chamber was also introduced into the design, in order to improve the reliability of the printhead.
Another trend in the industry is market demand for lower cost per print. Printhead producers can pack in greater ink volume per cartridge to increase the print count or install a permanent or semipermanent thermal printhead to reduce the cost of new ink cartridges. Again, such a trend demands even higher reliability for thermal ink-jet printheads.
Another popular model currently on the market comprises a 480-nozzle printhead. In the implementation, the 480-nozzle printhead consists of six colors with 80 nozzles per color.
Reference is now made to FIG. 11, which is a simplified diagram illustrating a piezoelectric print head comprising a piezoelectric nozzle 70 as discussed above. In the piezoelectric drop-on-demand ink-jet method, deformation of the piezoceramic material 72 causes the ink volume change in the pressure chamber to generate a pressure wave that propagates toward the nozzle 70. The acoustic pressure wave overcomes the pressure loss due to viscosity typical of a small nozzle. The wave also overcomes the surface tension force from the ink meniscus that forms so that an ink drop can begin to form at the nozzle. When the drop is formed, a pressure sufficient to expel the droplet toward a recording media must be exerted. The basic pressure requirements are shown in FIG. 12, which illustrates three different stages of drop formation, equivalent to the three stages shown in FIG. 8. At each stage a corresponding pressure is noted.
In general, the deformation of a piezoelectric driver is on the submicron scale. To have large enough ink volume displacement for drop formation, the physical size of a piezoelectric driver is often much larger than the ink orifice. Therefore, miniaturization of the piezoelectric ink-jet printhead has been a challenging issue for many years.
Independently from the thermal or piezo ink-jet method, bend or shear mode, one of the most critical components in a printhead design is its nozzle. Nozzle geometry such as diameter and thickness directly effects drop volume, velocity, and trajectory angle. Variations in the manufacturing process of a nozzle plate can significantly reduce the resulting print quality. Image banding is a common result from an out-of-specification nozzle plate. The two most widely used methods for making the orifice plates are electroformed nickel and laser ablation on the polyimide. Other known methods for making ink-jet nozzles are electro-discharged machining, micropunching, and micropressing.
Because smaller ink drop volume is required to achieve higher resolution printing, the nozzle diameter of printheads has become increasingly small. With the trends towards smaller diameters and lower cost, the laser ablation method has become popular for making ink-jet nozzles.
Print Head Registration and Lifetime Issues
Ink jet printing uses small nozzles as described above, that eject ink drops towards the print medium. The image is thus made of a huge number of ink drops—wherein the ink drop lands on the print medium. Each dot represents a pixel. The number of pixels or ink drops is very large compared to the number of ink jet nozzles, meaning that the firing frequency, the number of drops ejected per second, is very high. Typically around 10,000 drops per second are ejected from each nozzle during operation of a typical home ink jet printer. In addition there is a need to place the drops on the medium in a correct and very precise way in order to provide a good quality print image.
A typical way of transferring the ink is to mount the print head on a carriage and perform print scans back and forth over the print medium. During these print scans the location of the print head is determined precisely by encoders and the ink drops are placed on the medium as required.
Another way of transferring the ink to the print medium is to use the so-called full array method, concerning which see U.S. Pat. No. 4,477,823, the contents of which are hereby incorporated by reference. In the full array method a one-dimensional array is created such that there is full coverage of the pixels in one print line so that each nozzle relates to one pixel. Creating such a one-dimensional “full array” may be accomplished by a 2-D array due to the practical difficulties of building the necessary nozzle density in a single line.
With such a one-dimensional array, there is no need to mount the print head on a carriage since no side-to-side motion is needed. Furthermore due to the lack of side-to-side scanning, a much faster print speed is possible. Yet, the paper still needs to advance lengthwise for the next print line and thus there is still overall relative movement between the print medium and the nozzles, a fact that has inherent problems as will be described hereinbelow.
Ink Supply Issues
In order to eject the ink drops, ink channels supply ink to the print head from a main reservoir. In order to facilitate the supply, the pressure of the ink inside the ink jet nozzle has to be well regulated in order to achieve constant drop volume. Moreover, the ink pressure in the print heads used today is slightly lower than atmospheric pressure. These pressure conditions are crucial for drop ejection. The negative pressure is obtained by regulating the pressure inside the main reservoir using various methods such as pressure pumps, placing the reservoir below the print head, or capillary foam. Further details may be found in U.S. Patent Application No. 2001/043256, the contents of which are hereby incorporated by reference. Reference is made once again to FIG. 6, which shows how a drop is ejected when the pressure of trapped ink rises dramatically inside the ink chamber due to operation of the piezoelectric actuator 42.
The number of ink jet nozzles in a drop-on-demand print head is generally a few dozen, and the firing frequency is about 10,000 drops per second, implying that a very large number of drops are ejected in a single second for each one of the nozzles, leading to significant wear on the nozzle and the ejection mechanism.
The market demand is for faster printers with better print quality. To achieve faster printing it is necessary to increase the number of drops ejected per second. This can be done by raising the firing frequency and by enlarging the number of nozzles and indeed this is the technological trend in ink jet development. The trend is exemplified by International Patent Application No. WO03013863, the contents of which are hereby incorporated by reference. Printing at higher frequency dictates a faster movement between the ink jet nozzles and the print medium. This faster movement, naturally, is harder to control and the printer has to be more complex in order to support the movement of the carriage or the print medium. Achieving these two goals, that is higher firing frequency and greater number of nozzles, is inherently limited with the current ink jet technology as explained in the following.
Inherent printing problems of ink jet technology.
1. Chronic loss of operating nozzles: it is a common problem that while printing, some of the nozzles fail, that is they stop ejecting drops. In order to produce a drop, strict pressure and flow conditions inside the ink chamber part of the nozzle have to be maintained. Such maintenance can be problematic when both the number of ink jet nozzles and the firing frequency are increased.
Some of the factors that are responsible for the loss of operating nozzles are:                Sensitivity to vibrations, and to the acceleration and deceleration that are experienced when the print head carriage moves whilst printing. The faster the print head moves the worse such problems become and, as mentioned, a higher firing frequency dictates a faster print scan.        Air bubbles become trapped inside the ink supply system. Due to the physics of drop ejection, small air bubbles can penetrate into the ink jet nozzle and ink supply system. Such air bubbles can damage the ink jet nozzles' operation and ink supply.        Rapid changes in firing frequency create pressure waves inside the ink supply system due to variable ink consumption. The pressure waves change the ink pressure inside the ink jet nozzle, however it is important that the pressure remains constant in order to eject drops properly. The problem worsens when the total number of drops per second (firing frequency+number of ink jet nozzles) is increased.        
The loss of a single nozzle leads to the loss of many thousands of drops on the final image, directly impacting on the printing quality.
2. Satellite drops: Referring now to FIGS. 13 and 14, when ink drops are created by a print head they are typically not formed as single clean drops but rather as a large main drop and secondary smaller drops, also known as satellite drops. FIG. 13 is a series of photographs of drops being ejected from a nozzle. Each photograph in the series is taken at a different number of microseconds from drop ejection, and the series illustrates the evolution of main and satellite drops during the ejection process. FIG. 14 shows the effect of the main and satellite drops as the drops land on the print medium. Due to the relative motion between the print head and the print medium during printing, the main and satellite drops do not arrive at the same location on the print medium, but rather the satellite drops are displaced from the main drop landing point.
As described, conventionally, printing is carried out whilst the print head moves, that is during print scans. Because of the scan movement the main and satellite drops do not land at the same point on the print medium and this leads to undesired shapes of pixels at the printed image. Further discussion of the problem is available in European Patent Application No. 1,197,335, the contents of which are hereby incorporated by reference. The shape of the drops formed on the print medium directly influence print quality and the optimal drop shape is as round as possible. Obviously, the faster the print head moves the longer the “tail” or drop projection, on the print medium, as FIG. 14 clearly suggests.
The connection between pixel shape and print head speed implies that inherent deterioration of image quality happens precisely when increasing the speed of movement between the print head and the print medium, because of the distortion caused thereby to the drop shape. The loss of quality is irrespective of the technical difficulty of providing accurate control of the faster scan carriage.
3. Drop velocity & cross talk: As explained, printing is carried out during the course of relative movement between the print head and the print medium. Since the drop has to fly a fixed distance from the nozzle to the medium, its velocity determines the time it takes the drop to arrive at the medium. Due to the relative motion between s the print head and the print medium the time and thus the drop velocity affects the landing point of the drop on the print medium.
To make matters worse, there is an undesirable variance in drop velocity between the different nozzles within a single ink jet print head. Furthermore there is a cross-talk phenomena as well in that nozzles show a variation in their drop velocity due to operation of neighboring nozzles. The drop velocity variation is at least partly due to ink supply issues, and an ink supply method intended to reduce the problem, known as “center” feed design, is described in U.S. Pat. No. 4,683,481 to Johnson, the contents of which are hereby incorporated by reference. The disclosure, entitled “Thermal Ink Jet Common-Slotted Ink Feed Print head,” describes the use of small slots in the ink manifold. The slots serves as buffers that can absorb sudden pressure variations.
4. Wet on dry phenomena: the printed image comprises different parts which are not printed simultaneously. Consequently, there are regions where there is overlap between still wet or fresh drops and dry or old drops on the print medium. The fresh and old drops have different fluid characteristics that detract from simple and straightforward mixing of the inks in order to create the intended color, for example blue & yellow to create green.
Compared to visual display technology such as liquid crystal display (LCD) screens where an image is created instantaneously, ink jet printing is very slow. There is ongoing progress in ink jet printing speed, as disclosed, for example, in pat WO03013863, the contents of which are hereby incorporated by reference. Nevertheless the basic principal of printing remains the same—a print head launches drops of ink that lend on a print medium during relative motion therebetween, the relative motion being controlled in order to ensure that a given drop lands at an intended location. Conventional ink jet printing therefore cannot be instantaneous as it is dependent on the motion of a body having mass.
There is thus a widely recognized need for, and it would be highly advantageous to have, an ink jet printing system which is devoid of the above limitations.